Electrostatic spray coating system



Dec. 23, 1958 J. w. JUVINALL 2,865,789

ELECTROSTATIC SPRAY COATING SYSTEM Filed Feb. '7, 1955 I5 (J/'\ |5Q Wu 1 i 6 H 2 l4 l INVENTOR. JAMES W. JUW/VALL v BY flea-aw Alfarneys air-atomized enamel has met with a certain degree aura 2,865,789 ELECTROSTATIC SPRAY COATING SYSTEM James W. Juvinall, Indianapolis, Ind., assignor to Ransburg Electra-Coating Corp, Indianapolis, End, a corporation of Indiana Application February 7, 1955, Serial No. 486,638

7 Claims. (Cl. 11793) This application relates to improvements in spray coat- .ing and particularly :to the electrostatic coating of an article of .manufacture with porcelain .enamel sprayed in an .electrostatic field.

Porcelain enamel finishes for metal parts of stoves, laundry equipment, bathroom :appliances and other articles are known for their ability to withstand constant wear, ease of cleaning, and beauty. Such finishes are made by coating the article surface with finely ground frit and firing the coated article at temperatures in the order of 1000 F. and above to fuse the frit into a glass coating bonded .to the metal. Various methods of coating the article with frit prior to its firinghave been used.

The'application of porcelain enamel to articles by compressed air spray guns -is well'known. Inthe past some work has been done in using electrostatic forces to aid in depositing theair-atomized particles of enamel on the articles. An example of such work is reported in a threepart :article entitled Electrostatic Spraying of Porcelain -Enamel, by J. B. Willis in the June, July and August v1-945 issues of Finish magazine. As indicated in this article, the use of electrostatic forces to aidin depositing of success.

The usual porcelainenamel-orlike mixture for spraying is called a slip andiscomposed of a number of solid ingredients milled together to a-desired standard of fineness and a liquid vehicle for the solid ingredients. The

solid portion of the slip normally consists of frit combined with small porportions of a variety of chemical and mineral additives. These additivesare introduced to impart desired physical and chemical properties to either the slip, the dried coating (called the bisque) or the fired finish. The principal vehicle for milled enamel is water .to which maybe added small quantities of various liquids in order to-obtain some particular characteristics for the slip, bisque and/or'fired finish.

Atypical formulation of porcelain enamel to be applied 'Wiil'l air spray guns consists of 100 pounds of ,frit together with approximately 4 pounds of clay as a suspending agent and for imparting strength to the bisque,

. /2 pound of bentonite to harden the bisque and help reduce tearing of the fired finish, pound of sodium nitrite also to prevent tearing, and four ounces of gum tragacanth as a suspending agent. These materials are .milled Wet in a ball mill to a fineness so that 2% of the material would remain on a 200 mesh screen. Water is added to bring the specific gravity of the resulting slip to 1.65 and approximatelyten grams of urea per gallon of slip are added as a further measure to prevent tearing of the fired finish. Air spraying of such a slip produces a uniform bisque Without sag which when fired fuses into -a homogeneous finish without tearing, poppers, or'other ard by the American Society for flesting lviaterials-under ice as a thin film to the edge of a rotating disc maintained at high potential and the coating materialris atomized into finely divided spray particles and deposited in a quiescent atmosphere on the grounded articles under the influence of electrostatic forces. Quiescent is used in the sense that the atmosphere or air through which the charged particles move as they near deposition on the articles is free from air currents of such velocity and volume as to overcome, for many of the particles, the electrostatic attraction thereof and cause a substantial portion of the particles to escape deposition on the articles. This process of electrostatic coating in a quiescent atmosphere has been widely adopted in the conveyorized application of paints, synthetic enamels and lacquers to a variety of articles of manufacture because of its resulting improved coatings and great economies in painting costs.

Efforts to apply porcelain enamel with this airless electrostatic spraying process were generally unsuccessful. The formulation and physical properties of slips used in conventional compressed air spray guns were generally the same whether or not an electrostatic field was used to aid in depositing the enamel. However, when such slips were fed to a disc rotated at conventional speeds and maintained at;high potential, the enamel was not properly atomized in that the spray was composed of particles varying widely in size and generally too coarse to produce :the uniform bisque required. Difiiculties in electrostatically depositing such atomized particles were encountered. Wide departures from conventional formulations of the slip were tried without materially improving the quality of the atomization and'hence of the deposited enamel.

I have discovered that to obtain commercially satisfactory atomization and deposition of porcelain enamels, ceramics and other fusible (i. e., intended to be fused together into a substantially homogeneous finish by heat) materials in an electrostatic spraying process without the use of compressed air it is necessary to vary materially increase the fineness of grind of the solid ingredients in the slip. Such fine grinding results in a slip with superior properties particularly for airless electrostatic spraying, permits the formation of an excellent bisque on an article, and extends the advantages of airless electrostatic spraying to the ceramics industry. Increasing the fineness of grind of porcelain enamel also has advantages when the enamel is air sprayed in'an electrostatic field.

As previously mentioned, air sprayed enamel is conventionally milled'to a fineness measured by the percentage of the ground material remaining on a fine mesh screen. Most air sprayed enamels are milled so that two or three percent ofjthe solids are retained when the slip is passed through a 200 mesh (2G0 holes to the square inch) screen. .qui'red for satisfactory spraying without the use of com- I' have found that the fineness of grind repressed air cannot be properly measured by using such conventional measuring means. The material must be a ground so fine that no measureable amount (much less than .Ol%) remains on a 200 mesh screen.

Since conventional measuring means, that is, fine wire screens, do not permit satisfactory control of the fineness of grind of porcelain enamels manufactured in accordance with my discovery, itzhas been necessary to resort tornethods not previously required in measuring'the fineness of grind of porcelain enamels. One method which has beenv found particularly satisfactory for measuring and controlling the finenessof requir'edby my discovery involves the useofthe Fineness grind :of porcelain enamel of Grind Gauge. Thistgaugehas been adoptedfasstanddesignationDlZiOSA and the .use of the .gauge is ;described in detail in the Societys 1954 copyrighted pub- 3 lication entitled Standard Method for Testing for Fineness of Dispersion of Pigment-Vehicle Systems.

I have found particularly good results are obtained in the airless electrostatic spraying of porcelain enamel when the grind of the milled enamel is such as to produce -P. C. units and thus the optimum range of. fineness of grind contemplated by my invention is from 1.6 to 2.4 mils as measured on a Fineness of Grind Gauge. Due to the nature of the gauge as well as the properties of the material being measured, it cannot be assumed that readings on the mil scale of the Fineness of Grind Gauge can be translated directly into particle size of the solids in the material. Further, measurements on the Fineness of Grind Gauge are influenced to a certain extent by the specific gravity and certain other physical properties of the material being measured. Therefore, the Fineness of Grind Gauge measurements set forth herein were taken with slips having specific gravities in the range of approxi- Mils= -mately 1.60 to 1.65 and other conventional physical properties, although with certain slips it may be advisable to alter the specific gravity and/or other properties for the actual spray coating operation.

Milling together the various solid ingredients of a porcelain enamel slip in a ball mill does not, of course, reduce all ingredients to particles of uniform size and it is therefore difficult to express the fineness of grind of the enamel in terms of particle size. In the porcelain enamel industry it has been standard practice to express fineness of grind in terms of the percentage of solid materials retained by a 200 mesh screen. However, when 'the percentage retained is reduced to considerably below one percent, the readings are neither accurate nor do they properly distinguish between degrees of fineness in the range required.

By way of example, a given porcelain enamel of conventional fineness for air spray application was ground so that 2% was retained on a 200 mesh screen. Such grind was too coarse to be read on a Fineness of Grind Gauge and too coarse for commercially satisfactory airless electrostatic spraying. The same enamel was then ground so that .025 was retained on a 200 mesh screen and correspondingly 795% was retained on a 325 mesh screen. Such fineness of grind was still too coarse to register on a Fineness of Grind Gauge and still too coarse to produce proper results in an airless electrostatic spray process. When milled to a fineness of five P. C. units on a Fineness of grind Gauge to produce excellent results in an airless electrostatic spray process, no measurable residue was retained on a 325 mesh screen and, of course, none on a conventional 200 mesh screen.

A disadvantage of fine grinding of porcelain enamels is an increased propensity of the enamel to tear or exhibit other undesirable characteristics during firing. Tearing is an uneven distribution of the various ingredients of the enamel while in semi-liquid state at high temperatures during the firing operation and manifests itself in an unsightly coating through which can be seen patches of the blue-black ground coat beneath the white cover coat of enamel. However, the proper use of certain electrolytes known to the art such as sodium nitrite, sodium thiosulphate, sodium sulfocyanate, bentonite and area will prevent tearing and other difficulties during firing 4 with porcelain enamels and like materials milled to the fiineness required by my discovery. The term antitearing electrolyte is used herein to include those additives just mentioned as well as other materials which may be used to prevent tearing of the fired finish.

In the accompanying drawing, Fig. 1 illustrates apparatus which has been successfully used for coating the tops of automatic home washing machines with porcelain enamel in accordance with my invention; and

Fig. 2 illustrates details of the atomizing device shown in Fig. l.

A succession of washing machine tops 10 are suspended from and moved at a uniform predetermined rate by a conveyor 11. A portion of conveyor 11 is in helical form as shown so that the washing machine tops 10 are moved in a helical path around an atomizing device shown generally as 14. The atomizer includes a rotatable disc 15 having a smooth fiat upper surfacewhich terminates in a circular edge 15a preferably of knifelike sharpness. Disc 15 is rotated about its central axis by means of a motor located within motor housing 16. Disc edge 15a is maintained approximately 27 inches from the washing machine tops 10 and the motor and disc are positioned axially of the helical portion of conveyor 11 by means of support 17 constructed at least in part of a suitable insulating material. The disc is 20 inches in diameter and is rotated at a speed of approximately 3500 R. P. M.

0m porcelain enamel slip used with the illustrated apparatus has the following formulation: 100 pounds of frit, 3 'pounds of clay, 1 pound of bentonite, pound of sodium nitrite, pound potassium chloride, A pound sodium aluminate, and ounce of gum tragacanth, with 4 ounces of urea per gallon of slip and water added to bring the specific gravity of the mixture to 1.65. This slip is milled in a ball mill to a fineness represented by five P. C. units on a Fineness of Grind Gauge. The porcelain enamel slip, contained in a reservoir 20, is fed at a controlled rate by means of a positive displacement plunger type pump 21 through tube 22 to the center of the upper surface of disc 15 at the rate of approximately 250 cc. per minute.

A potential difference in the order of kilovolts is established in the quiescent atmosphere between the edge of disc 15 and the washing machine tops 10 by connecting the high voltage terminal of a high voltage source 25 to atomizer 14 which is isolated from ground by insulating support 17 and by grounding the other terminal of source 25 and the washing machine tops as shown.

The porcelain enamel slip fed to the rotating disc spreads over its surface and is atomized from adjacent edge 15a into spray particles of satisfactory fineness. The atomized particles of porcelain enamel are electrically charged and move through the electrostatic field established between edge 15a of the disc and the grounded washing machine tops 10 and are dispersed and deposited onto the washing machine tops under the influence of the electrostatic field. When the enamel is milled to a fineness in the order of that represented by five P. C. units on a Fineness of Grind Gauge, the enamel is properly atomized from the disc and deposited on the washing machine tops to form an excellent bisque which when fired will fuse into a very satisfactory cover coat of enamel with no tearing or other undesirable characteristics. However, if this same porcelain enamel were milled to a conventional fineness as represented by 2% being retained on a 200 mesh screen, it would not atorn-' ize properly and would not produce the desired bisque.

While there have been shown and described certain embodiments of my invention, it should be understood it is capable of various modifications. Thus changes may be made without departing from the spirit and scope of the invention which is disclosed in the appended claims.

Ielaim:

1.- In a method of applying a porcelain enamel finish to an article, the steps of atomizing a porcelain enamel slip whose fineness of grind is represented by in the order of 5 P. C. units measured on a Fineness of Grind Gauge and containing an anti-tearing electrolyte, and in a quiescent atmosphere electrostatically depositing the atomized porcelain enamel on the article.

2. The method of applying porcelain enamel to an article, comprising feeding porcelain enamel slip at a controlled rate to the surface of a rotating disc, said slip having a fineness of grind represented by in the order of 5 P. C. units measured on a Fineness of Grind Gauge, containing an anti-tearing electrolyte and having a specific gravity of substantially 1.65, moving the article to be coated along a predetermined path in coating relation with said disc, creating an electrostatic field of high potential gradient between the disc and the moving article, atomizing the slip from said disc into said electrostatic field to form an electrically charged spray of dispersed porcelain enamel particles, and electrostatically depositing the porcelain enamel particles onto the article to form a bisque, said atomization and deposition being carried out in a quiescent atmosphere.

3. The method of coating an article, comprising the steps of atomizing a liquid containing fusible ceramic particles along an extended atomizing zone remote from the article to be coated, said fusible ceramic particles having a fineness of grind represented by at least A: P. C. units on a Fineness of Grind Gauge and containing an anti-tearing electrolyte, creating a high electrical poten tial difference between the atomized material and the article, and electrostatically depositing the atomized material on an article in a quiescent atmosphere.

4. In a method of applying a fused finished coating on an article, the steps of atomizing a liquid containing fusible coating material particles whose fineness of grind is represented by at least ,4, P. C. units measured on a Fineness of Grind Gauge and also containing an antitearing electrolyte, and electrostatically depositing the atomized liquid containing solid particles on the article.

5. A material for spray coating articles comprising a liquid containing fusible coating material particles having a fineness of grind represented by at least P. C. units measured on a Fineness of Grind Gauge and also containing an anti-tearing electrolyte.

6. A porcelain enamel slip having a fineness of grind in the order of 5 P .C. units measured on a Fineness of Grind Gauge and also containing an anti-tearing electrolyte.

7. The method of coating an article, comprising grinding a liquid containing fusible ceramic particles to a fineness of grind represented by at least P. C. units on a Fineness of Grind Gauge, adding an anti-tearing electrolyte to the liquid, feeding said liquid to an atomizing zone remote from the article to be coated, creating a high electrical potential difference between the liquid at said zone and the article, atomizing the liquid at said zone into a spray, and electrostatically depositing the atomized spray on the article.

References Cited in the file of this patent UNITED STATES PATENTS 2,431,629 Wind et a1. Nov. 25, 1947 2,537,956 Baldwin Jan. 16, 1951 OTHER REFERENCES Manual of Porcelain Enameling (The Enamelist Publishing Co., Cleveland, Ohio), copyright 1937 (p. relied on).

Ransburg: N0. 2 Process, The Electrostatic Paint Spray, Ransburg Electro-Coating Corp., Indianapolis, Ind., 1953-117-EP (pages 6 to 9). 

2. THE METHOD OF APPLYING PORCELAIN ENAMEL TO AN ARTICLE, COMPRISING FEEDING PORCELAIN ENAMEL SLIP AT A CONTROLLED RATE TO THE SURFACE OF A ROTATING DISC, SAID SLIP HAVING A FINENESS OF GRIND REPRESENTED BY IN THE ORDER OF 5 P.C. UNITS MEASURED ON A FINENESS OF GRIND GAUGE, CONTAINING AN ANTI-TEARING ELECTROLYTE AND HAVING A SPECIFIC GRAVITY OF SUBSTANTIALLY 1.65, MOVING THE ARTICLE TO BE COATED ALONG A PREDETERMINED PATH IN COATING RELATION WITH SAID DISC, CREATING AN ELECTROSTATIC FIELD OF HIGH POTENTIAL GRADIENT BETWEEN THE DISC AND THE MOVING ARTICLE, ATOMIZING THE SLIP FROM SAID DISC INTO SAID ELECTROSTATIC FIELD TO FORM AN ELECTRICALLY CHARGED SPRAY OF DISPERSED PORCELAIN ENAMEL PARTICLES, AND ELECTROSTATICALY DEPOSITING THE PORCELAIN ENAMEL PARTICLES ONTO THE ARTICLE TO FORM A BISQUE, SAID ATOMIZATION AND DEPOSITION BEING CARRIED OUT IN A QUIESCENT ATMOSPHERE. 